Scanning type non-destructive testing systems have heretofore been provided for detecting defect indications. For example, the O'Connor et al U.S. Pat. No. 3,744,030 and the Flaherty et al U.S. Pat. No. 3,774,162 disclose systems in which a laser beam is impinged on an oscillating mirror system to produce scanning movement over the surface of a part while light from the surface of the part is detected by photocell means to develop electrical signals indicating the position of defect-indicating material on the surface of the part. The defect-indicating material may be a penetrant material or it may include magnetic particles in dry form or in a liquid carrier, applied on the surface of a magnetizeable part during or after magnetization thereof. In either case, a fluorescent material may be associated with the defect-indicating material. As disclosed in the aforesaid O'Connor et al patent, the fluorescent material may be such as to respond to the wavelength of the laser beam to produce high energy emissions at a second wavelength and the detector may be arranged to have maximum sensitivity at the second wavelength. As a result, a high contrast is obtained, with high sensitivity and resolution.
Such prior systems have also included pattern recognition systems which are especially advantageous in certain applications such as, for example, in applications in which elongated cracks having a certain orientation are of primary interest.
With such systems, high satisfactory and reliable results have been obtained in many applications and the rejection of unsatisfactory parts has been insured with a very high degree of reliability. However, deficiencies of such systems have not been recognized. For example, it has not been appreciated that such systems may sometimes operate to indicate that a part should be rejected when, in fact, the part is satisfactory.